Stacks of alternating high and low refractive index layers serve as mirrors in Vertical Cavity Surface Emitting Lasers, hereinafter referred to as VCSELs. The task is to find suitable materials for the high and low index layers which maximize a ratio of the high index refractive order to the low index refractive order, and which could be deposited in a manner compatible with the semiconductor device processing.
A VCSEL is attractive as a device in which the lasing cavity is perpendicular to the top surface of a laser chip, which is small and which may be produced by planar technology. This can lead to a promising future in high density laser arrays, high data transmission in optical communication systems, ultra high parallel processing in optical communication systems, as well as supplying a route for data transmission between electronic chips. Furthermore, the circular-like nature of their beams allows one to efficiently couple the laser light into circular optical fibers.
In the VCSEL the light output is in the film growth direction which is usually parallel to the direction of the injection current. Due to this feature, the mirror through which the emission takes place and the electrical contact physically occupy the same side of the laser structure, i.e. either the top or the bottom of the device. The mirror is located approximately in the center of the surface while the electrode is located peripherally of the mirror. An example of a surface emitting laser with a coplanar mirror/electrode arrangement in which a gold layer with a thickness of a few tenths of a micrometer acts as the mirror through which laser-emission takes place, may be found in articles by H. Soda et al., entitled "GaInAsP/InP Surface Emitting Injection Lasers," Japanese Journal of Applied Physics, Vol. 18, No. 12, 1979, pp. 2329-2230; and by H. Soda et al. entitled "GaInAsP/InP Surface Emitting Injection Lasers with Short Cavity Length," IEEE Journal of Quantum Electronics, Vol. QE-16, No. 6, Jun. 1983, pp. 1035.1041. However, S. Kinoshita pointed out that such mirrors lead to low quantum efficiency primarily due to absorption of lasing emission by the gold mirror and suggested the use of a stack of pairs of dielectric layers as the top mirror, one layer of each pair having a higher index of refraction than the other layer of the pair. See an article by Susumu Kinoshita et al. entitled "GaAlAs/GaAs Surface Emitting Laser with High Reflective TiO.sub.2 /SiO.sub.2 Multilayer Bragg Reflector," Japanese Journal of Applied Physics, vol. 26, No. 3, March 1987, pp. 410-415; L. M. Zinkiewicz et al., "High Power Vertical-Cavity Surface-Emitting AlGaAs/GaAs Diode Lasers," Appl. Phys. Letters, Vol. 54, No. 20, 15 May 1989, pp. 1959-1961; and Kenichi Iga, "Recent Advances of Surface Emitting Semiconductor Lasers," Optoelectronics-Devices and Technologies, Vol. 3, No. 2, December 1988, pp. 131-142.
TiO.sub.2 and ZrO.sub.2 quarter-wave ##EQU1## dielectric layers have been typically paired with SiO.sub.2 quarter-wave layers. The number of pairs is selected to obtain a maximum performance reflectivity. However, the mirror structure of alternating TiO.sub.2 (or ZrO.sub.2) and SiO.sub.2 quarter-wave layers have not yielded expected performance, in terms of reflectivity. Theoretically, the optical performance of a stacked mirror structure should approach 100 percent. Unfortunately, presently obtainable performance falls within a broad range of from 90 to 99 percent. The problem resides, primarily, with the high index layer materials. This shortfall is, most likely, due to the difficulty in obtaining sufficiently high quality TiO.sub.2 (or ZrO.sub.2) layers on a reproducible basis. Electron-beam deposition of coatings, such as TiO.sub.2 (or ZrO.sub.2), requires addition of oxygen in the deposition process to get the proper stoiciometry for a desired refractive index. Addition of oxygen is needed to avoid formation of unwanted, oxygen-deficient phases, such as Ti, TiO, Ti.sub.2 O.sub.3, Ti.sub.3 O.sub.5, which occur due to an oxygen shortage. This requirement makes it difficult to reproducibly form the TiO.sub.2 layer.
Several single crystal semiconductors with high index of refraction, such as Al.sub.x Ga.sub.1-x As or GaInP, which possess the desired properties when epitaxially deposited may be used in place of TiO.sub.2 or ZrO.sub.2 layers; however, the epitaxial growth of these materials requires temperatures of .about.600.degree.-800.degree. C. along with sophisticated, expensive growth apparatus. These materials are poorly suited for deposition in a device post-processing wherein temperatures above 300.degree.-350.degree. C. are to be avoided. Therefore, there is still a need for high stability, high performance mirrors for use in VCSELs with high quality coatings which are easily reproducible at conditions compatible with the device processing and which could be also produced in a simplified manner utilizing planar technology.